Make core/ includes absolute, remove subfolders from include path

This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.

modules/dds/texture_loader_dds.cpp

@@ -29,7 +29,7 @@
 /*************************************************************************/
 
 #include "texture_loader_dds.h"
-#include "os/file_access.h"
+#include "core/os/file_access.h"
 
 enum {
 	DDS_MAGIC = 0x20534444,